This invention relates to a lens holder driving device and, in particular, to an autofocus lens holder driving device for use in a compact camera.
Mobile terminals such as camera-equipped cellular mobile phones almost mount compact cameras therein. The compact camera is provided with an autofocus lens holder driving device. Previously, various autofocus lens holder driving devices have been proposed.
By way of illustration, Japanese Unexamined Patent Application Publication No. 2009-251031 (JP-A-2009-251031) (which will be also called Patent Document 1) discloses a lens driving device which is capable of decreasing a parts count and of miniaturizing. The lens driving device disclosed in Patent Document 1 comprises a lens holder in which a lens assembly can be mounted, a driving coil fixed to the lens holder at outside circumstance thereof, a permanent magnet opposite to the driving coil, an outer yoke including the permanent magnet, an elastic member (an upper elastic member and a lower elastic member) for supporting the lens holder in a direction of an optical axis of the lens assembly shiftably, and a base disposed at a lower side of the lens holder. In Patent Document 1, the elastic member (the upper elastic member and the lower elastic member) is disposed in the inside of the outer yoke. The outer yoke comprises an outer hollow cylindrical portion having an inner wall in which the permanent magnet is provided and a ring-shaped end portion disposed at an upper end of the outer hollow cylindrical portion. The upper elastic member is mounted to the outer yoke via a spacer. The base, the lens holder, and the spacer are made of plastic molded parts.
Incidentally, a lens holder driving device as disclosed in the above-mentioned Patent Document 1 moves the lens holder up or down along the direction of the optical axis by flowing a current through the driving coil.
In a case where no current flows through the driving coil, an lower end portion of the lens holder goes and makes contact with an upper surface of the base because the lens holder is urged or energized downwards by the elastic member. In this event, the lens holder has a moving distance (a stroke) (μm) which is equal to 0 μm (or a minimum stroke). The 0 μm (or the minimum stroke) is called an “infinity (INF) position” of the lens holder driving device.
On the other hand, when a current flows through the driving coil, the lens holder gradually moves upward linearly along the direction of the optical axis against a downward urging force of the elastic member. More specifically, when the current flowing through the driving coil goes beyond a first predetermined current value (mA), the lower end portion of the lens holder goes away from the upper surface of the base. The first predetermined current value will be referred to as a lower regulated current value. In addition, when the current flowing through the driving coil reaches a second predetermined current value (mA), an upper end portion of the lens holder goes and makes contact with an inner wall surface of the ring-shaped end portion of the outer yoke. In this event, the moving distance (the stroke) (μm) of the lens holder is equal to a maximum stroke. The maximum distance corresponds to a position which gets near a subject from a “macro position” of the lens holder driving device. In addition, the second predetermined current value will be referred to as an upper regulated current value.
In addition, the macro position is a close-in position for picking up, as the subject, an identifier such as a two-dimensional bar code or the like, and corresponds to a position where a position (a focal distance) between a lens of the camera and the two-dimensional bar code (the subject) is, for example, about 10 cm. On the other hand, the infinity (INF) position is an infinite position for picking up the subject which substantially located at an infinite position, and corresponds to a position between the position (the focal distance) between the lens of the camera and the subject is infinity.
In the manner which is described above, by flowing, through the driving coil, a current between the lower regulated current value and the upper regulated current value, it is possible to move the lens holder to any position between the infinity (INF) position and the macro position. Under the circumstances, it is desirable that the lens holder moves linearly along the direction of the optical axis so as to proportionate to the current value of the current flowing through the driving coil.
However, the present inventor found out that the following malfunctions (problems) occur upon moving the lens holder up to the vicinity of the maximum stroke by making the current flow through the driving coil.
More specifically, in spite of the fact that the current flowing through the driving coil does not reach to the upper regulated current value, there is a case where the upper end portion of the lens holder goes and makes contact with the inner wall surface of the ring-shaped end portion of the outer yoke because the lens holder is sucked to the inner wall surface of the ring-shaped end portion of the outer yoke in the vicinity of the maximum stroke. On the other hand, in spite of the fact that the current flowing through the driving coil makes less than the upper regulated current value, there is a case where the upper end portion of the lens holder does not go away from the inner wall surface of the ring-shaped end portion of the outer yoke and remains in a state where the upper end portion of the lens holder makes contact with the inner wall surface of the ring-shaped end portion of the outer yoke. That is, the present inventor discovered the malfunctions (the problems) so that a range capable of ensuring linearity becomes narrow in a driving response of the lens holder driving device and a margin in the maximum stroke become lost.